Gangliosides are sialic acid containing glycosphingolipids composed of a complex carbohydrate moiety linked to a hydrophobic ceramide portion. Embedded within the outer leaflet of the cell membrane, the carbohydrate chain is exposed to the extracellular matrix. Qualitative and quantitative changes in ganglioside composition during cell differentiation and proliferation have been observed and seem to reflect the state of malignant transformation of cancers of neuroectodermal origin (Hakomori 1985). Malignant melanoma cells express a variety of complex gangliosides in addition to GM3, the major ganglioside in normal melanoctyes (Carubia et al. 1984). Altered ganglioside metabolism in melanoma causes additional expression of GD3, GD2, GM2,9-O-Acetyl-GD3 and GT3 (Hamilton et al. 1993; Tsuchida et al., 1987). Treatment of patients with anti-GD3 monoclonal antibodies resulted in inflammation at the tumor site and partial regression of metastasis was seen occasionally, suggesting, that gangliosides are suitable targets for immune attack (Houghton et al., 1985). The generation of human MAb's reactive with GD3 from melanoma patients (Yamaguchi et al., 1987) support the idea, that gangliosides are potential immunogens as well.
In studies aimed at inducing a humoral response against gangliosides in melanoma patients by active immunization, GM2/BCG vaccines seemed to be most effective (Livingston et al., 1987; Livingston et al., 1989). In a randomized study with 122 melanoma patients, who were disease-free after surgery, that it was showed that, out of 64 patients treated with BCG alone and 58 patients with GM2/BCG, the majority of patients (86%) receiving the GM2 vaccine produced antibodies. Patients that produced anti-GM2 antibodies had a significantly longer disease free and overall survival than antibody negative patients. Comparing the two arms of the trial, patients receiving the GM2/BCG vaccine had a 17% improvement in disease-free interval and 9% improvement in survival when compared to the BCG control group, though neither result was statistically significant (Livingston et al., 1993a). Unfortunately, the immune response was only of short duration, mostly IgM and of moderate titer. This suggested that GM2 was recognized as a T-cell independent antigen as a consequence of carbohydrate antigens (Livingston et al., 1989) and also because gangliosides are auto antigens expressed on some normal tissue (Hamilton et al., 1993). Similar approaches with GD2 and 9-0-Acetyl-GD3 vaccines in patients resulted in occasionally low titers and no antibody response against GD3 could be detected (Livingston, 1991).
New potent adjuvants were able to enhance the immune responses against gangliosides in some cases, but especially for auto antigens such as for tumor associated gangliosides a different approach had to be utilized. Based on Landsteiner's classical experiments (Landsteiner and Chase, 1942) with hapten-carrier conjugates, covalent attachment of poorly immunogenic antigens to immunogenic carrier proteins has been successfully used to enhance immune response. For example responsiveness to carbohydrates, other than gangliosides, could be accomplished with conjugation to appropriate carrier proteins. Coupling of bacterial capsular polysaccharides to immunogenic proteins showed a significant increase in immune response and protection (Eskola et al., 1990). Recently, vaccination of ovarian cancer patients with synthetic Thompson Friedenreich tumor antigen conjugated to keyhole limpet hemocyanin elicited humoral IgM and IgG response (MacLean et al., 1992). The important finding common in these studies was the isotype switch from a IgM response of short duration to a long lasting, high affinity IgG response indicating that activation of T-cell dependent pathways against carbohydrates is likely to occur. This approach is now applied to the melanoma tumor antigen GD3 to develop a method to synthesize ganglioside-protein conjugate vaccines and examine the immunogenicity of different GD3-protein conjugates in mice.